A faint flash from the cosmic dawn has revealed itself as the oldest supernova ever confirmed, its light having traveled 13 billion years to reach Earth. The James Webb Space Telescope captured the event that happened when the universe was only 730 million years old-only 5% of its present age-and was triggered by a rare gamma-ray burst designated GRB 250314A at the extraordinary redshift of 7.3.
The detection unfolded through a coordinated chain of observatories. The French–Chinese SVOM satellite first detected the high-energy gamma rays on March 14. NASA’s Neil Gehrels Swift Observatory picked up the same event in X-rays ninety minutes later, thanks to its precise localization. Eleven hours after Swift’s alert, the Nordic Optical Telescope in the Canary Islands picked up the faint optical afterglow as the blast wave plowed into surrounding gas. Four hours later, the European Southern Observatory’s Very Large Telescope measured the spectrum of the afterglow, confirming the extreme redshift and placing the explosion firmly in the first billion years of the universe.
A redshift this high imposes a striking observational effect: cosmic time dilation stretches the apparent evolution of distant events. Whereas the light curve of a nearby supernova peaks in brightness over several weeks, the light curve from this ancient detonation was stretched to roughly three and a half months in our rest frame. With this prediction in hand, astronomers used Director’s Discretionary Time on JWST to observe the field precisely when the supernova should be brightest.
On July 1, JWST’s NIRCam directly imaged the supernova glow: unambiguous proof that GRB 250314A was indeed the death of a massive star. “Only Webb could directly show that this light is from a supernova a collapsing massive star,” Andrew Levan (Radboud University and the University of Warwick) said. The telescope also resolved the host galaxy-a tiny smudge of light spanning a few pixels, but already showing similarities to other galaxies from the same epoch.
Surprisingly, the supernova’s spectrum proved very familiar: though exploding in an epoch where heavy elements were far less abundant, its profile closely resembles modern core-collapse supernovae. That would mean at least some massive stars in the Epoch of Reionization-when ultraviolet light from young stars was ionizing the universe’s hydrogen fog-evolved and died in ways similar to stars today. Subtle differences may yet appear with deeper analysis, especially in line strengths influenced by the low metallicity environment.
Gamma-ray bursts, like GRB 250314A, are believed to form when rapidly rotating, massive stars collapse into black holes and launch relativistic jets that beam intense radiation across the cosmos. Their rarity at such early times makes them invaluable probes of early star formation. Long-duration GRBs act as “beacons,” enabling astronomers to study stars individually and their surroundings, even if host galaxies are too faint for direct spectroscopy.
JWST’s detection was enabled by its near-infrared sensitivity, crucial for high-redshift work since the expansion of the cosmos redshifts ultraviolet and visible light from early galaxies into infrared wavelengths. At z = 7.3, the supernova’s rest-frame optical emission is stretched past 2 microns, where the instruments on JWST shine. The observation showcases further a key point about multi-epoch strategies: time-dilation in the prediction of the peak means that the supernova was caught at maximum brightness, hence maximizing the scientific return.
This find breaks the record of JWST’s most distant supernova, seen so far at 1.8 billion years after the Big Bang, and attests to the telescope’s capability to connect transient phenomena with their progenitors across great expanses of time. “This observation also demonstrates that we can use Webb to find individual stars when the universe was only 5% of its current age.” said Levan. With more observing time granted, the team plans to capture additional ancient GRBs and their afterglows; thus, it enables a clearer picture to be built of massive star deaths during the universe’s formative centuries.
